In many situations it is desired to expand a tube radially within a surrounding structure, thereby anchoring the tube in the desired position and forming a leak proof joint between the tube and the surrounding structure. An older traditional form of swaging is known as roller swaging, in which an implement is inserted in the tube and, as it rotates, gradually deforms the tube outwardly. However, roller swaging, while still in common use, is time consuming and is characterized by a tendency to reduce the thickness of the tube wall with accompanying elongation of the tube. It can also cause work hardening, stress induced corrosion cracking and fatigue. Preferable swaging techniques are hydraulic swaging in which fluid pressure is applied within the tube to produce radial expansion, as described in U.S. Pat. No. 4,502,308, and draw bar swaging in which elastomeric material is compressed axially, causing it to expand radially within the tube, as described in U.S. Pat. No. 4,387,507.
By properly swaging a tube, a permanent leak proof joint that is not readily subject to corrosion can be formed by eliminating spaces between the tube and the surrounding structure. A mechanical interlock can be formed between the tube and the surrounding structure to insure that the tube will not be pulled loose, even if the joint should begin to leak. The formation of a highly secure mechanical interlock is particularly important in, for example, the boiler of a ship in which many tubes pass through a tube sheet.
Frequently, a short length of tube projects from the primary side of the tube sheet and is readily accessible. This tube end is sometimes flared or belled at the primary side where the tube ends to prevent the tube from moving toward the secondary side of the tube sheet. The present invention, however, relates to the creation of an interlock at the less accessible side of the joint, normally the secondary side of the tube sheet. Here, a positive visually verifiable interlock can be created by causing an exposed portion of the tube to bulge or expand permanently to an outside diameter greater than the bore in which the tube is located. This technique is required by the U.S. Navy, which demands a bulge extending 3/8 inches from the secondary side of the tube sheet.
It should be understood that the supported portion of the tube within the tube sheet or other surrounding structure can be, and frequently is, subjected to internal pressure substantially in excess of that required to burst an unsupported tube. The exposed and unsupported portion of the tube, beyond the face of the tube sheet, that is to be expanded must be limited to a significantly lower pressure. This is relatively easily accomplished by roller swaging.
An objective of the present invention is to provide an improved joint swaging technique using hydraulic forces to produce a visually verifiable mechanical interlock at the less accessible side of the joint. A further objective is to insure that no corrosion receptive crevices remain between the tube and the surrounding structure at the surface of that structure.